Field of the Invention
The present invention relates to a light emitting diode lamp (LED lamp) that is efficiently restricted from emitting, for instance, visible light rays.
Background Art
Light emitting diodes have advantages that the power consumption thereof is lower and the life thereof is longer compared to well-known incandescent lamps (e.g., halogen lamps). With enhancement in awareness of ecology by demanders, the usage fields of the light emitting diodes have been rapidly expanding as one of the measures for energy saving. Especially, the light emitting diodes have been increasingly used as relatively compact light sources used as sensors and so forth.
For example, Japan Laid-open Patent Application Publication No. 2013-186095 discloses a technology for a light emitting diode lamp to cut off visible light rays from light rays emitted from a light emitting diode with use of a band-pass filter.
Incidentally, as to an AlGaAs infrared light emitting diode used as an infrared sensor or so forth, emission wavelength distribution is likely to be elongated to a visible light side although the wavelength of light, corresponding to the peak of the amount of light emission, reliably falls within a wavelength range of infrared light. Hence, visible light rays are also included in light rays emitted from the AlGaAs infrared light emitting diode.
When used as a sensor, a light emitting diode lamp is likely to be preferred to emit light rays in which visible light rays (red light rays) are not included so as not to make a viewer perceive whether or not the light emitting diode lamp is lit.
In general, a band-pass filter is used to cut off light rays with unnecessary wavelengths. With use of the band-pass filter, the light emitting diode lamp used as a sensor can also cut off a large part of visible light rays.
However, it was found that the band-pass filter tends to be unable to exert a light blocking (cutoff) function with respect to light rays incident on the band-pass filter at incident angles of greater than a predetermined angle (this tendency will be hereinafter referred to as “incident angle dependency” of the band-pass filter). Moreover, it was also found that this tendency is remarkable for a type of band-pass filter in which an optical thin film is disposed on the surface of a substrate. A boundary wavelength for determining whether or not light rays should be cut off is more definitely set for the band-pass filter with the optical thin film than for, e.g., a type of band-pass filter that selects light rays allowed to transmit therethrough by absorbing unnecessary light rays. Hence, there is high demand to use the band-pass filter with the optical thin film.
Furthermore, in general, “divergence angle” of light rays emitted from a light emitting diode is definitely presented in such a condition as sale of the light emitting diode. For example, when the divergence angle of light rays emitted from the light emitting diode is 10 degrees, this means that 50% of the total amount of light rays emitted from the light emitting diode form angles of less than or equal to 10 degrees together with the optical axis of the light emitting diode. In other words, the remaining 50% of the total amount of light rays form angles of greater than 10 degrees together with the optical axis of the light emitting diode. By taking this point into consideration together with the aforementioned incident angle dependency of the band-pass filter, resultant conclusion is that when a light emitting diode lamp is obtained by simply combining a light emitting diode and a band-pass filter, a large amount of light rays are supposed to be incident on the band-pass filter at incident angles of greater than the maximum incident angle, up to which the band-pass filter is capable of exerting the light cutoff function. Because of this, a drawback has frequently emerged that light rays with undesired wavelengths cannot be completely cut off in spite of using the band-pass filter.
The present invention has been developed in view of the aforementioned drawback of the well-known art. Therefore, it is a main object of the present invention to provide a light emitting diode lamp that can reduce, as much as possible, chances of emitting light rays with undesired wavelengths in use of a band-pass filter having specific incident angle dependency.